1. Field of the Invention
The present invention relates to methods for detecting optical data, optical devices for detecting optical data, and reading-writing apparatuses for optical data which are appropriately used for reading from and writing to media such as optical discs and magneto-optical discs. In particular, the present invention relates to a method for detecting optical data, an optical device for detecting optical data, and a reading-writing apparatus for optical data, in which, in a focus detecting system which uses a spot-size method using a holographic element, an optical element for increasing the size of a light spot formed on a photo-detector by zeroth diffraction order from the holographic element is provided, and tracking errors can be detected by a method such as a DPP (differential push-pull) method.
2. Description of the Related Art
In a reading-writing apparatus for reading and/or writing optical data from and/or on an optical medium such as an optical disc, tracking control for positioning light from a light source on a recording track of the recording medium and focusing control for focusing the light at a recording face of the recording medium are performed. An optical device for detecting optical data for performing such controls is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2000-11398.
In this optical device, light from a laser 60 is collimated by a collimator lens 61, and the collimated light is applied to a diffraction grating 62, as shown in FIG. 1. Light polarized in a given polarizing plane of zeroth diffraction order and ±1 diffraction orders diffracted by the diffraction grating 62 are transmitted by a polarization beam splitter 63 and are applied to a ¼-wave plate 64. The light applied to the ¼-wave plate 64 is converted from linearly polarized light into circularly polarized light, and the circularly polarized light is applied to a recording face of an optical disc 66 through an objective lens 65.
The light reflected from the recording face of the optical disc 66 is collimated by the objective lens 65. The collimated light is converted from circularly polarized light into linearly polarized light by the ¼-wave plate 64 in another polarizing plane differing by an angle of 90 degrees from the polarizing plane in which the light from the laser 60 is applied. The light emitted from the ¼-wave plate 64′ is reflected by the polarization beam splitter 63 toward a focusing lens 67. The reflected light is condensed by the focusing lens 67, and is applied to a holographic element 68 provided with, for example, off-axis Fresnel zone plates. The light applied to the holographic element 68 is thereby diffracted. The holographic element 68 functions as a lens to dispose the focal points of the ±1 diffraction orders along the optical axis of the zeroth diffraction order with the focal point of the zeroth diffraction order being between the focal points of the ±1 diffraction orders.
The light diffracted by the holographic element 68 is applied to a roof prism 69 at the bottom face thereof. The roof prism 69 is disposed accurately such that the vertex of the roof prism 69 coincides with the optical axis of the zeroth diffraction order from the holographic element 68.
The ±1 diffraction orders and the zeroth diffraction order are emitted from the roof prism 69, each being divided into two light beams. The light is radiated to a photo-detector 70 disposed at the focal point of the zeroth diffraction order from the holographic element 68 and the focusing lens 67.
That is, the light from the laser 60 is radiated to the photo-detector 70 by being divided into, for example, the zeroth diffraction order and the ±1 diffraction orders.
The light is applied to the photo-detector 70 in the pattern shown, for example, in FIG. 2.
In FIG. 2, the zeroth diffraction order from the holographic element 68 corresponding to the zeroth. diffraction order from the diffraction grating 62 is divided into two components by the roof prism 69 and forms six light spots 6a to 6f on the photo-detector 70. The ±1 diffraction orders from the holographic element 68 corresponding to one of the ±1 diffraction orders from the diffraction grating 62 is divided into two components by the roof prism 69, thereby forming semicircular light spots 6g to 6j on the photo-detector 70.
The focal points of the ±1 diffraction orders from the holographic element 68 are offset from each other along the optical axis of the zeroth diffraction order with the focal point of the zeroth diffraction order being between the focal points of the ±1 diffraction orders, whereby the ±1 diffraction orders are defocused. Therefore, the light spots formed with the ±1 diffraction orders, which are circular, are each divided into two semicircular spots. The ±1 diffraction orders from the holographic element 68 corresponding to the other one of the ±1 diffraction orders are not shown in the drawing because they are not used by the photo-detector 70.
Tracking errors and focusing errors are detected with the light spots being formed on the photo-detector 70, as described above.
The photo-detector 70 is provided with photo-detecting parts 6A to 6F for detecting the light spots 6a to 6f, respectively, photo-detecting parts 6G to 6I for detecting the light spots 6g and 6h, and photo-detecting parts 6J to 6L for detecting the light spots 6i and 6j. Tracking control is performed by, for example, determining variations in the amount of light applied to the light spots 6a to 6f, and focusing control is performed by, for example, determining variations in the area of the light spots 6g to 6j. 
In the above known optical device for detecting optical data, since the six light spots 6a to 6f are formed on the photo-detector 70 and the tracking errors are detected by using six determination signals, tracking errors can be easily detected by using a plurality of light spots, as in a DPP method or the like.
The light fluxes which are applied to the photo-detector 70 are each divided into two components by the roof prism 69, whereby the shapes of the light spots for detecting focusing errors, which are formed with the ±1 diffraction orders from the holographic element 68, differ from each other, as shown in FIG. 2. When the objective lens 65 focuses on the optical disc 66, the light spots 6i and 6j formed on the photo-detecting parts 6J to 6L separate from each other in a radial direction of the optical disc 66, and the light spots 6g and 6h formed on the photo-detecting parts 6G to 6I overlap each other, in which the light spots 6i and 6j and the light spots 6g and 6h are not symmetrical. Therefore, there is a problem in that displacement of the component parts of the optical device for detecting optical data easily affects the accuracy of detection of focusing errors.
There is another problem in that the photo-detector 70 tends to be large because the photo-detecting parts 6J to 6L, which receives light spots having an area larger than that of the photo-detecting parts 6G to 6I for the light spots 6g and 6h which overlap each other, must be used for detection of focusing errors and be positioned away from the photo-detecting parts 6A to 6F which receive the zeroth. diffraction order so that the light spots are prevented from overlapping each other.
Moreover, an optical component part such as the roof prism is required for splitting light fluxes, and the optical component parts must be formed with high accuracy and be mounted accurately because the shape and the position of the optical component parts determine the split position of the light fluxes.